Figure 3: Electron diffraction reveals the sharp onset of cation exchange.

For NCs subject to 0 Cu⁺ ions per NC and 384 Cu⁺ ions per NC respectively (a,b), selected area electron diffraction patterns are indicative of the CdSe wurtzite (cadmoselite) structure. As shown in Table 1, the innermost ring at 3.55 Å is a combination of the (100), (002) and (101) planes, which are Debye–Scherrer broadened into one ring. The three outer rings, measured at 2.15, 2.00 and 1.88 Å, match those for the (100), (103) and (200) planes of cadmosellite. In addition, there is a ring corresponding to the (102) plane, at a lattice spacing of 2.55 Å. These features are seen for samples (a–c). At 1,317 Cu⁺ ions per NC (in slight excess of the stoichiometric amount needed for full conversion), the tetragonal Cu₂Se phase (bellidoite) is seen (e). The highest intensity reflections include ones at 3.46 Å for the (113), (311) and (222) planes, 2.04 Å for the (404) plane, and 1.76 Å for the (226) and (533) plane. The diffraction features of Cu₂Se seem to first appear at 549 Cu⁺ ions per NC (c), overlapped with CdSe diffraction features. As exchange progresses, CdSe diffraction features wane, until a pure bellidoite diffraction pattern is obtained for the fully converted sample. All d-spacings are determined relative to the (111) reflection of an Au standard taken under the same conditions. Samples were prepared by a slow titration of CdSe NCs in toluene with increasing amounts of a methanolic solution of Cu⁺, similar to the titration experiments described in Methods. At various points in the titration, a small aliquot was drop-casted onto an ultrathin carbon transmission electron microscope grid and washed with methanol several times. The total amount of Cu⁺ added is indicated in Table 1 in the form of number of ions per NC, estimated as described in Methods.